Cross-contaminant shield in sputtering system

ABSTRACT

A multi-chamber processing system is described for depositing materials on multiple workpieces (wafers, display panels, or any other workpieces) at a time in a vacuum chamber. The system includes a sputtering chamber and a separate pre-clean chamber, where wafers can be transferred between the two chambers by a robotic arm without breaking a vacuum. The wafers are mounted one-by-one onto a rotating pallet in the pre-cleaning chamber and sputtering chamber. The pallet is firmly fixed to a rotatable table in the sputtering chamber. Copper tubing in the table couples RF energy to the wafers, and a liquid running through the copper tubing controls the temperature of the wafers. Multiple targets, of the same or different materials, may concurrently deposit material on the wafers as the pallet is rotating. Multiple magnets (one for each target) in the magnetron assembly in the sputtering chamber oscillate over their respective targets for uniform target erosion and uniform deposition on the wafers. An electrically insulated target backing plate between each magnet and a target has a liquid channel running through it for controlling temperature. The distance between the magnets and the targets is made very small by a thin aluminum plate fixed to the bottom segment of the target backing plate by a dip brazing process. Various shields are described to prevent cross-contamination from the targets and prevent the sputtered target material from entering gaps in the chamber and shorting out insulators.

FIELD OF THE INVENTION

This invention relates to deposition systems for semiconductor wafersand other workpieces and, in particular, to sputtering systems.

BACKGROUND

A sputtering system is widely used in the semiconductor manufacturingindustry for depositing materials on semiconductor wafers. Sputtering issometimes referred to as physical vapor deposition, or PVD. In asputtering operation, thin films comprising materials such as Al, Au,Cu, Ta are deposited in a vacuum on silicon wafers or other substrates.To make a stack of thin films of different materials, one commonpractice is to use multiple single-wafer process chambers wherein eachchamber deposits only one material on one substrate at a time. For atri-layer structure comprising Ti/Cu/Au, three separate single-waferprocess chambers are required to sequentially deposit Ti, Cu and Au ontothe substrate. The dedication of an individual process chamber tospecific material deposition adds equipment costs and processing costs.The limitation of single-wafer transfer between chambers slows down thesystem throughput.

Various other drawbacks are found in typical sputtering systems. Thesedrawbacks relate to inefficient magnetron operation, non-uniformcoverage, poor temperature control of the wafers, contamination of thewafers and other components, and non-uniform target erosion, among otherthings.

SUMMARY

A multi-chamber batch processing system is described for depositingmaterials on multiple workpieces in a vacuum chamber.

The system includes a sputtering chamber and a separate pre-cleanchamber, where wafers can be transferred between the two chambers by arobotic arm without breaking a vacuum. In one embodiment, 4-6 wafers areprocessed at a time in each chamber.

Having a separate pre-clean chamber, such as an inductively coupledplasma (ICP) chamber, increases throughput, provides a faster etch rate,results in less contamination of the sputtering chamber, and results inless damage to the electronic circuits on the wafer.

Further, since there is no need for a shutter for isolating sputteringtargets during the pre-clean process, there is no added contaminationfrom a shutter during the sputtering process.

The wafers are mounted one-by-one from a load lock to a rotating palletin the ICP chamber. In one embodiment, the pallet is aluminum. Thealuminum is anodized (or another insulator is formed) to provide aninsulating film on its surface. This prevents the pallet from beingetched in the ICP chamber, avoiding particulates from the palletcontaminating the wafers.

The robotic arm transfers the wafers one-by-one to the sputteringchamber from the ICP chamber without the wafers being exposed to theatmosphere, thus avoiding undesirable chemical reactions on the wafersurface, e.g: oxides.

The wafers are mounted on a direct-drive rotating pallet in thesputtering chamber. The pallet is firmly fixed to a rotatable table inthe sputtering chamber to provide good thermal and electricalconductivity between the pallet and the table. Copper tubing in thetable couples RF energy to the wafers, and a coolant running through thecopper tubing controls the temperature of the wafers.

Multiple targets, of the same or different materials, may concurrentlydeposit material on the wafers as the pallet is rotating. This enableshigher throughput, creates a uniform deposition, and can be used todeposit films of varying compositions on the workpieces.

Multiple magnets (one for each target) in the magnetron assembly in thesputtering chamber oscillate (0.5-10 second period) over its associatedtarget for uniform target erosion and uniform deposition on the wafers.Each magnet is composed of many small magnets, whose arrangement andrelative sizes are selected to optimize the target erosion and increasethroughput.

A target backing plate between each magnets and a target has a coolantchannel running through it. The distance between the magnets and thetargets is made very small by a thin aluminum plate fixed to a bottomsegment of the target backing plate by a dip brazing process. This smalldistance increases the magnetic coupling and thus density of plasma,leading to improved deposition rates and target utilization

Various shields are described to prevent cross-contamination from thetargets and prevent the sputtered target material from entering gaps inthe chamber and shorting out insulators.

Other novel features of the system are described.

The system may also be used for depositing material on LCD panels (e.g.,conductors for a thin film array) and other workpieces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the multi-chamber sputtering andcleaning system with covers removed to show some internal components.

FIG. 2 is a top down view of the multi-chamber system exposing therotating pallets and robotic arm in the transport module.

FIG. 3 is a cutaway view of the sputtering chamber.

FIG. 4A is a cross-sectional view of the rotating shaft, table, andpallet in the sputtering chamber.

FIG. 4B is a bottom view of the table showing a copper tube for RFcoupling and coolant flow.

FIG. 5A illustrates the distribution of some magnetic flux lines in thepermanent magnet used in the magnetron.

FIG. 5B is a cross-sectional view of the magnet of FIG. 5A.

FIG. 6A is a perspective view of the magnetron assembly, forming a topportion of the sputtering chamber, with the oscillating magnet in amiddle position.

FIG. 6B is a perspective view of the magnetron assembly with oneoscillating magnet at its leftmost position.

FIG. 7 is a perspective cross-sectional view of a portion of the topplate and target backing plate (for supporting the target and magnets)of the sputtering chamber.

FIG. 8 is a top down view of the target backing plate without its thincover, showing a coolant channel.

FIG. 9 is an exploded cross-sectional view illustrating a dip brazingprocess for forming the target backing plate between the magnet and thetarget.

FIG. 10 is a cross-sectional view of the top plate and target backingplate with a mounted target.

FIG. 11 is a perspective view of the underside of the top plate showingthe targets and cross-contamination shields.

Elements with the same numbers in the various figures are the same.

DETAILED DESCRIPTION

FIG. 1 illustrates a multi-chamber sputtering and pre-clean system 10for workpieces such as semiconductor wafers, LCD panels, and otherworkpieces requiring the deposition of thin films. Examples of thinfilms include Al, Cu, Ta, Au, Ti, Ag, Sn, NiV, Cr, TaNx, Hf, Zr, W, TiW,TiNx, AlNx, AlOx, HfOx, ZrOx, TiOx, and alloys of two or more of theseelements. The top covers of the sputtering chamber 12, pre-clean chamber14, and wafer transport module 16 have been removed. The robotic arm inthe wafer transport module 16 is not shown in FIG. 1 in order to see theaccess ports 18, 19 from load locks 20, 21.

Typical wafer sizes are 6, 8, and 12 inches, and the system iscustomized for the particular workpieces for processing.

FIG. 2 is a top down view of the system 10, where the wafer-supportingpallets are revealed in chambers 12 and 14. The robotic arm 24 is shownin the transport module 16.

To load wafers into the system 10 for thin film deposition, a stack ofwafers supported in a cassette is placed in load lock 20. The cassettesupports each wafer by its edge. A vacuum is then created in load lock20 and transport module 16 by a vacuum pump. The vacuum pumps used inthe system can create pressures below 0.001 milli-torr.

The robot arm 24 rotates to align itself with the load lock 20, and arm24 is inserted into load lock 20 by rotation of an arm 26. The cassetteis positioned by an elevator so that the bottom wafer is slightly abovearm 24. The elevator then lowers the cassette so that the wafer issupported entirely by arm 24. Arm 24 is then pulled back into thetransport module 16, and arm 24 is aligned with port 28 of the pre-cleanchamber 14. The pre-clean chamber 14 is isolated from the transportmodule 16 by a slit valve (not shown). The pressure in the pre-cleanchamber 14 is brought down by a vacuum pump 29 (FIG. 1) to the samepressure (base pressure) as in the transport module 16, and the slitvalve is opened. Arm 24 extends the wafer over a rotatable pallet 30 inchamber 14. Pallet 30 rotates to align a wafer support area 32 below thewafer. The wafer support area 32 is an indented area in pallet 30 sizedto accommodate the particular wafers being processed. In anotherembodiment, electrostatic chucks (ESC) are used to support the wafer. AnESC provides added flexibility in biasing the wafer, and each ESC may beseparately controlled. An ESC may also provide better thermalconductance between the wafer and the pallet 30 due to a strong clampingaction on the wafer.

Four pins below pallet 30 are raised to extend through four holes 34 inthe wafer support area 32 to lift the wafer off arm 24. Arm 24 iswithdrawn, and the pins are lowered so that the wafer is seated in theindentation and the entire back surface of the wafer is in contact withpallet 30. This is important for temperature control and biasing, to bedescribed later.

The robotic arm 24 then goes back to get another wafer from thecassette, and the pallet 30 rotates to align the next wafer support area32 with the port 28. The transfer process is repeated until five wafersare placed on pallet 30. In a preferred embodiment, pallet 30 has fourto six wafer support areas 32 but there may be more or fewer areas asdesired.

During the process of loading the pre-clean chamber 16, arm 24 may alsobe removing cleaned wafers from pallet 30 and placing them on a similarpallet 36 in the sputtering chamber 12. The sputtering chamber 12 has aport 37 and slit valve similar to those of the pre-clean chamber 14. Theloading process onto pallet 36 is the same as the loading describedabove.

Pre-cleaning of the wafers is important to remove impurities, e.g:oxides, from the wafer's surface so that metal films deposited in thesputtering chamber are not electrically insulated from the wafer. Byperforming pre-cleaning in chamber 14, part of a multichamber vacuumenvironment to which sputtering chamber 12 is connected, the wafers canbe transported from the cleaning chamber 14 to the sputtering chamber 12without being exposed to the atmosphere (or otherwise contaminated), soimpurities do not form on the workpiece during the transportation time.Further, vacuum pump-down cycles are reduced since a vacuum ismaintained in the multi-chamber system during transfer of the cleanedwafers to the sputtering chamber. Only when a cassette is full in loadlock 21 or when a cassette is empty in load lock 20 does the system needto break the vacuum to remove or introduce wafers from and to thesystem.

In some sputtering systems, the pre-clean is performed in the samechamber as the sputtering (in situ). This results in a compromiseddesign of the equipment and causes etched particles to accumulate on thechamber walls and other portions. Such particles contaminate the waferduring the sputtering process and shorten time between maintenancecycles. Further, since there is no need for a shutter for isolatingsputtering targets during the pre-clean process, there is no addedcontamination from a shutter during the sputtering process.

In the preferred embodiment, the pre-clean chamber 14 uses inductivelycoupled plasma (ICP) for etching the wafer. A coil 38 (FIG. 1) at thetop of chamber 14 is energized with an external RF source (e.g., at13.56 MHz) to create an excitation field in the chamber 14. Argon gasflows through the chamber 14 from an external gas source. The argonatoms in the chamber 14 are ionized by the RF energy so are charged. Thewafers are biased by a DC biasing source coupled to the aluminum pallet30 so that the ions are attracted to the wafers and etch the wafers.Other gases may be used depending on the desired etch rate and materialsto be etched. The etching is a cleaning process rather than a process toetch features in the wafer materials so the energy levels may be low.This avoids damaging circuit devices and features already formed in thewafer. ICP etching is a well known process so additional detail is notnecessary for describing the chamber 14 and its operation.

The aluminum pallet 30 in the pre-clean chamber 14 is anodized toprovide an electrically insulating film on its surface. This reduces theetch rate of pallet 30 when the wafers are being cleaned in the ICPchamber, avoiding particulates from the pallet contaminating the wafers.The anodized surface may be obtained by heating pallet 30 in an oxygenatmosphere, depositing a layer of aluminum oxide, or plasma spraying alayer of aluminum oxide. An insulating surface of pallet 30 may also beobtained by depositing a ceramic coating, or other insulating films. Athicker insulating film reduces the effective bias at the pallet surfaceand thus reduces the etching rate of the pallet. In one embodiment, theinsulating film is greater than 2 mils (0.05 mm).

In another embodiment, a material may be deposited on pallet 30 that isdesired to be deposited on the wafers during the pre-clean process. Theplasma clean will then dislodge the material from pallet 30 and coat thewafers with the material.

After the pre-clean process, in which any unwanted native oxide has beenetched off the wafer surface, the robot arm 24 transports the fivewafers, one-by-one, into the sputtering chamber 12. FIG. 3 is a cutawayview of the sputtering chamber 12 with its cover removed. The belowdescription of the pallet 36 and table 40 in the sputtering chamber 12also applies to the pallet 30 and table in the pre-clean chamber.

FIG. 3 illustrates pallet 36 mounted on a rotatable table 40. Pallet 36and table 40 may be formed of aluminum. Pallet 36 may be continuouslyrotated at any speed or may be temporarily stopped to control thedeposition of a sputtered material from a target 43 overlying a wafer. Awafer 41 is shown in one of five wafer supporting areas 32.

Pin bellows 39 is shown in FIG. 3 for pushing up the four pins (notshown), described above, into the wafer support area 32 for transferringthe wafer to and from the robotic arm 24. The pin bellows 39 may becontrolled pneumatically or driven directly by a motor.

A chamber shield 35 prevents contaminants from accumulating on thechamber wall.

FIG. 4A is a cross-sectional view of pallet 36 and table 40. Pallet 36is about ¼ to ½ inch (6.3-12.7 mm) thick, and table 40 is about 1 inch(25.4 mm) thick. Pallet 36 is a single piece that is fixed to table 40by a countersunk screw 42 at the indentation in each wafer support area32 so that the wafers block the sputtered materials being deposited onscrews 42. Pallet 36 may be removed for cleaning by unscrewing screws42.

The entire back surface of each wafers is thus in electrical and thermalcontact with pallet 36, which is in turn in electrical and thermalcontact with table 40.

Controlling the temperature of the wafers is important during thesputtering process to obtain a predicable and reliable thin film. Thetemperature of the wafers is controlled by flowing a coolant 44 (FIG.4A) through a copper tube 46 in direct contact with table 40. In oneembodiment, the copper tube 46 is brazed to table 40. The copper tube 46runs in a groove 48 around the table 40, as shown in FIG. 4B, which is abottom view of table 40.

The copper tube 46 extends up through a rotating shaft 49 attached totable 40.

An external cooling source 50 cools the coolant (e.g., water) andrecycles the coolant back to table 40. Flexible tubing 51 from thecooling source 50 attaches to a rotatable coupler 52 for providing asealed coupling between the rotating copper tubes 46 (input and output)and the stationary tubing 51 to/from the cooling source 50.

In another embodiment, the cooling source may be replaced or augmentedby a heating source to increase the workpiece temperature independent ofthe ICP or sputtering process.

An RF and bias source 54 is electrically coupled to the copper tube 46by the rotatable coupling 52 to energize table 40 and thus energizepallet 36 and the wafers for the sputtering process. In anotherembodiment, table 40 is grounded, floated, or biased with only a DCvoltage source.

When the chamber 12 is evacuated and back filled with a certain amountof Ar gas at a certain pressure (for example, 20 milli-torr) and the gasis energized with a DC source, an RF source, or a combination of the twosources, an electromagnetic field is coupled inside chamber 12 to excitea sustained high density plasma near the target surface. The plasmaconfined near the target surface (described later) contains positiveions (such as Ar⁺) and free electrons. The ions in the plasma strike thetarget surface and sputter material off the target. The wafers receivethe sputtered material to form a deposited layer on the surface of thewafers. In one instance, up to twenty kilowatts of DC power can beprovided on each target. In such a case, each target can depositapproximately 1 micron per minute of copper, simultaneously, on multiplework pieces.

The chamber 12 wall is typically electrically grounded in processingoperations.

A bias voltage on the wafers can drive a flux of an electrically chargedspecies (Ar⁺ and/or atomic vapor sputtered off the target) to thewafers. The flux can modify the properties (for example, density) of thesputtered material to the wafers.

Generating a plasma for sputtering and the various biasing schemes arewell known, and any of the known techniques may be implemented with thedescribed sputtering system.

In a preferred embodiment, the chamber gas is provided by a distributionchannel at the bottom of the chamber 12, rather than from the top, whichreduces particle contamination during the sputtering process and allowsoptimization of the magnetron assembly (described later).

FIG. 3 illustrates a motor 58 for rotating shaft 49. Shaft 49 isdirectly coupled to the motor 58 so that pallet 36 is directly driven bymotor 58. This greatly increases the accuracy of positioning pallet 36over a belt drive or a gear drive. In the embodiment of FIG. 3, motor 58surrounds shaft 49 and has a central rotating sleeve fixed to shaft 49.Motor 58 may be a servo or stepper motor. In one embodiment, the motoris a servo motor that uses an absolute encoder attached to shaft 49 todetermine the angular position of shaft 49. In an absolute encoder, adisc with fine optical markings uniquely identifies the angular positionwithout 5 the need for counting pulses or determining a home position.For example, the disc may be glass covered with an opaque film with anumber of etched concentric rings taking the form of different lengthtransparent dashes. The set of light openings at each radial positionacross the rings creates a unique digital code. A motor controller,using an LED and phototransistors, senses the optical markings at eachradial position and uses that information to position shaft 49 for waferloading and unloading and to control the RPM of pallet 36 during thedeposition process (typically 5-30 RPM).

A seal 57 provides a seal around shaft 49 in order to maintain a lowpressure in chamber 12.

The sputtering chamber 12 uses a magnetron assembly, outside the vacuum,to further control the bombardment of the target by the plasma. In atypical system a fixed permanent magnet is located behind the target(serving as a deposition source) so that the plasma is confined to thetarget area. The resulting magnetic field forms a closed-loop annularpath acting as an electron trap that reshapes the trajectories of thesecondary electrons ejected from target into a cycloidal path, greatlyincreasing the probability of ionization of the sputtering gas withinthe confinement zone. Inert gases, specifically argon, are usuallyemployed as the sputtering gas because they tend not to react with thetarget material or combine with any process gases and because theyproduce higher sputtering and deposition rates due to their highmolecular weight. Positively charged argon ions from the plasma areaccelerated toward the negatively biased target and impact the target,resulting in material being sputtered from the target surface.

FIG. 5A illustrates one of the three magnets 60 overlying a targetbacking plate 59 (FIG. 3), supported by a grounded top plate 62 (FIG. 3)in the sputtering system. Magnet 60 has a triangular or delta shape withrounded corners. In one embodiment, the thickness of magnet 60 isbetween 0.5-1 ¼ inch thick (12-31 mm). In the example of FIG. 5A, thereare three rings (nested patterns) of individual magnets 63, whereadjacent rings have opposite poles so that a magnetic field spans acrossone ring to the next. Some magnetic field lines 64 are shown. Sincethere are three rings of magnets, there are two racetracks of fieldlines. These magnetic fields pass through the target backing plate 59and intersect the target 43 attached to the underside of the targetbacking plate 59 in FIG. 3. The plasma density at the target (and thusthe erosion rate) is greatest at the highest magnetic field intensity.The sizes, shapes, and distribution of the individual magnets 63 areselected to create a uniform erosion of the target, as described below.

FIG. 5B is a cross-sectional view of one embodiment of the magnet 60.The magnets 63 are mounted to a magnetic backing plate 65, also known asa shunt plate, formed of a ferrous material. The shape and magneticproperties of the shunt plate 65 may be altered to optimize theperformance of magnet 60.

The magnet 60 may also be an electromagnet.

FIG. 6A illustrates the magnetron portion of the sputtering chamber 12,where one magnet 60 is shown above a target (not shown). Two otheridentical magnets would be located above two other targets centered at120 degree intervals. A servo motor 66 using an absolute encoder,similar to the motor 58 for the shaft 49 that rotates table 40, iscontrolled by a motor controller to oscillate the three magnets 60 backand forth in unison over their associated targets at an oscillatingperiod of between 0.5-10 seconds. The magnets 60 are oscillated so thatthe magnetic fields are not always at the same position relative to thetarget. By distributing the magnetic fields evenly over the target,target erosion is uniform.

If the oscillation is too slow, then there may be time for particles ofone material to accumulate on portions of a target of a differentmaterial in areas not subjected to the magnetic fields for a prolongedperiod. When the magnet eventually scans over that portion of thetarget, the sputtered material will undesirably constitute the mixedmaterials (varying the stochiometry of the sputtered material). The0.5-10 second period is adequate for stochiometrically sensitivereactive films sputtered onto the workpiece. The oscillation period maybe slower for non-stochiometrically sensitive reactive films sputteredonto the workpiece.

An insulating bracket 67 secures each magnet 60 to motor 66 so thatthere is a minimum gap between the oscillating magnet 60 and the targetbacking plate 59.

Since there is no field in the middle portion of magnet 60, the magnet60 must scan a distance of at least half its width (and preferablyalmost its entire width) so that the middle portion of the targetexperiences the same magnetic fields as other portions of the target.

The individual magnets 63 along the edge of magnet 60 are smaller thatthe inner magnets so that the magnetic field extends close to the edgeof magnet. The span of a magnetic field can be approximated by thedistance between the centers of the two opposite poles. Hence, thediameters of the outer magnets 63 are made small (e.g., 0.5-1 cm). Theinner rings of magnets 63 may be larger. In the example, the innermagnets 63 are rectangular to shorten the distance between the innermagnets and the outer magnets.

The size of magnets 60 depends on the size of the wafers, whichdetermines the size of the targets. In one embodiment, a magnet 60 isabout 10.7 inches (27 cm) long and about 3 inches (7.6 cm) wide at itwidest part. An eight inch wafer may use a target that is from 10-13inches long in the radial direction. A twelve inch wafer may use atarget that is from 13-18 inches long in the radial direction. Thesetarget and magnet length dimensions are very small compared to the priorart. These small dimensions mean more efficient chamber volume, thus asmaller footprint; and also smaller and more efficient targets,resulting in lower costs for the targets and system. Generally, thetarget and magnet length perpendicular to the scanning direction isbetween 1.1 and 1.5 times the smallest dimension of the workpiecesurface facing the target.

FIG. 6A illustrates magnet 60 at a middle position during anoscillation, while FIG. 6B illustrates magnet 60 at a leftmost positionduring the oscillation.

To maximize the magnetic fields around the target, the distance betweena magnet 60 and a target should be minimized. Further, the targetbacking plate 59, with magnet 60 on one side and a target on the otherside, needs to be cooled due to the hot plasma in the chamber 12.

FIG. 7 is a perspective cross-sectional view of the top plate 62 andtarget backing plate 59, both formed of aluminum. The thickness of thetop plate 62 is about 1 ⅜ inch (35 mm). The magnet 60 oscillates in therecessed area 70 of the target backing plate 59. The target 72 isapproximately the shape of the recessed area 70 and is secured to thetarget backing plate 59 by solder, a braze, a conductive epoxy, copperdiffusion, or other known technique.

The target backing plate 59 (comprising the recessed area 70 and theraised area 74 around the recessed area 70) and the target 72 areelectrically connected to a negative bias voltage source in order forthe plasma to be concentrated in the area of the target 72. A wire (notshown) carrying a negative bias voltage is connected by a screw to theraised area 74 using one of the screw holes 75. Target 72 is alsoreferred to as the cathode, since it is negatively biased. The top plate62 supporting the target backing plate 59 is electrically grounded. Aninsulator ring 76 (e.g., a synthetic rubber ring, or other elasticmaterial) electrically insulates the target backing plate 59 from thegrounded portion. The ring 76 also mechanically supports the targetbacking plate 59. It is important to prevent conductive sputteredparticles from contacting ring 76 to prevent a short between the targetbacking plate 59 and the grounded portion.

The thickness of the recessed area 70 (the distance between magnet 60and target 72) should be thin to maximize the magnetic coupling to thetarget 72. In one embodiment, the thickness is between 0.5-0.75 inch(12.7-19 mm). The top of the recessed area 70 is a thin aluminum plate78 (e.g., 0.7-3 mm) that is dip brazed to the bottom segment 80 of therecessed area 70. Between the plate 78 and bottom segment 80 is acoolant (e.g., water) channel 82, shown in FIG. 8. A heated liquid mayalso be run through channel 82.

FIG. 8 illustrates a simplified channel 82 formed in the bottom segment80 between a coolant input port 84 (also see FIG. 7) and a coolantoutput port 86. Flexible tubing (not shown) connects each port 84/86 foreach magnet to an external coolant source so that each recessed area 70is independently cooled by a coolant flowing through the channel 82. Asshown in FIG. 8, the channel 82 is in a serpentine pattern and in ashape such that the area of the channel 82 varies based on the amount ofcooling needed to maintain the same temperature over the entire recessedarea 70. In one embodiment, the thickness of the coolant channel 82 is1-3 mm. The coolant enters at the wide portion of the target backingplate 59 where the temperature is normally the hottest.

The plate 78 that forms the top surface of the recessed area is dipbrazed to the bottom segment 80 as follows. As shown in FIG. 9, a thinaluminum alloy foil 88 (a eutectic material) having the general shape ofthe bottom segment 80 is interposed between the bottom segment 80 andthe thin plate 78. The eutectic foil 88 has a melting point lower thanthe melting temperature of the aluminum used to form the bottom segment80 and thin plate 78. Various eutectic aluminum alloys can be used. Aclamp presses the thin plate 78 and the bottom segment 80 together, andthe structure is placed in a molten salt bath at a temperaturesufficient to melt the eutectic foil 88 but not hot enough to melt purealuminum. The melting of the eutectic sheet 88 brazes the thin plate 78to the bottom segment 80.

In one embodiment, the aluminum used for the bottom segment 80 and thethin plate 78 is designated as 6061 aluminum in the trade and has amelting temperature of 1110 degrees F. (598 degrees C.). The materialused for the eutectic foil 88 is designated as 4047 aluminum in thetrade and has a melting temperature of 1065 degrees F. (574 degrees C.).The composition of 4047 aluminum is: Si 11.0-13.0%; Cu 0.3%; Mg 0.1%; Fe0.8%; Zn 0.2%; Mn 0.15%; A1 remainder.

FIG. 10 is a cross-sectional view of a portion of the grounded top plate62, insulator ring 76, target backing plate 59 (comprising the thinplate 78 dip brazed to the bottom segment 80), and target 72. A groove87 for a gas seal gasket is formed in the top plate 62. The magnet 60and coolant channels 82 are not shown.

The target 72, since at a negative bias, must not contact the groundedtop plate 62. Therefore, there must be a gap between the target 72 andthe top plate 62. If this gap is small enough, it creates a dark spacewhere there is not enough space to create a plasma. Sputtered particlesfrom target 72 entering the dark space will accumulate on the insulator76 and eventually short the target backing plate 59 to the top plate 62.Given the manufacturing tolerances in the sizes of the top plate 62 andthe target 72, as well as the mounting of the target 72, it is verydifficult to ensure that the gap between the target 72 and the top plate62 will be a minimum. To close up the dark space and preventcontamination of the insulator 76, a thin top shield 90 is fixed to thetop plate 62 by countersunk screws. FIG. 10 shows the resulting narrowgap 91.

FIG. 11 is a perspective view of the underside of the top plate 62 andtargets 72, showing the top shield 90. Since the top shield is thinaluminum or stainless steel and easily stamped to a precise tolerance,the gap between the target 72 and the top plate 62 is easily set to be aminimum (e.g., 1-2 mm). The thickness of the top shield 90 is in therange of 3/16-¼ inch (4.7-6.3 mm). The diameter of the top plate 62 isabout 28 inches (71 cm). In prior art systems, a separate anode ring forthe dark spaces was installed and aligned after the gap between thetarget and top plate was first determined.

The top shield 90 also overlies all otherwise-exposed portions of thetop plate 62 to prevent the top plate 62 from accumulating sputteredmaterial. (The exposed edge of the top plate 62 in FIG. 11 rests on topof the chamber wall (shown in FIG. 3) so is not exposed in the chamber.)When cleaning of the chamber 12 is necessary, the top shield 90 iseasily removed from the top plate 62 (by screws 94 in FIG. 11) andcleaned or disposed of. Typically, the top shield 90 will be cleaned upto ten times and then thrown away. This avoids the much more complex jobof removing the top plate 62 for cleaning. Accordingly, a single,inexpensive top shield is used for both creating the dark spaces andprotecting the top plate 62.

FIG. 11 also shows cross-contamination shields 96 between each target 72position for preventing sputtered material from one target from beingdeposited on a wafer that is not directly under the target as the pallet36 (FIG. 3) rotates. FIG. 3 shows a portion of a cross-contaminationshield 96. The vertical walls of cross-contamination shields 96 shouldbe less than 10 mm, and preferably less than 3 mm, from the top of thewafers (or other workpieces). The height of the cross-contaminationshields 96 depends on the height of the top plate 62 above the wafers,but will normally be about 1-6 inches (2.5-15 cm).

The described sputtering system allows for all three targets toconcurrently sputter the same or different materials on the wafersduring a batch process. This increases throughput and allows thesputtering of alloys or layers on the wafers without breaking a vacuum.To select an alloy composition, one target may be one material, and theother two targets may be a second or third material. For depositingstacked layers of distinct materials, then only one material may bedeposited at a time (e.g, one target energized at a time or multipletargets of the same material energized at a time). For depositing mixedlayers (e.g. alloys of distinct materials), then all targets may beenergized at the same time, assuming the targets are of differentmaterials.

More targets and wafers than shown in the examples may be employed inthe system. For example, there may be eight targets. The number of suchtargets is limited only by the ability to build increasingly narrowmagnets, which deliver a suitable magnetic flux on the target surface.

The tables/pallets on which the wafers are placed may be equipped withheaters to heat the wafers if desired. Heating may be created byresistive heaters mounted to the table or flowing a heated fluid throughthe copper tubing 46 (FIG. 4A). Such heaters are well known. Resistiveheaters are described in U.S. Pat. No. 6,630,201, incorporated byreference.

The system sputters approximately 240 Å of A1 for an input energy of 1kW-minute. In contrast, a typical prior art system, all other conditionsbeing equal, deposits approximately 90 Å of A1 for an input energy of 1kW-minute.

Conventional aspects of the system that have not been described indetail would be well known to those skilled in the art. U.S. Pat. No.6,630,201 and U.S. patent application Publication 2002/0160125 A1 areincorporated herein by reference for certain conventional aspectsprimarily related to creating a plasma and supplying gas to a processchamber.

Although the system has been described with respect to forming a metalfilm on semiconductor wafers, the system may deposit any material,including dielectrics, and may process any workpiece such as LCD panelsand other flat panel displays. In one embodiment, the system is used todeposit materials on multiple thin film transistor arrays for LCDpanels.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit and inventiveconcepts described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

1. A sputtering system comprising: a low pressure chamber; a pallet inthe chamber, the pallet having a plurality of workpiece support areasfor concurrently supporting a plurality of workpieces on the pallet; agenerally circular first plate above the pallet, the first platesupporting a plurality of targets having sputtering surfaces facing thepallet; the first plate forming at least a portion of a wall for thechamber for maintaining a low pressure in the chamber; and a pluralityof cross-contamination shields fixed to the first plate and extendingtoward the pallet, a cross-contamination shield being located betweenadjacent targets, the cross-contamination shields at least partiallyshielding workpieces not directly beneath a particular target fromsputtered material from that particulat target.
 2. The system of claim 1further comprising a shield plate fixed to the first plate, the shieldplate having openings for the targets, the shield plate shielding thefirst plate during a sputtering process, the cross-contamination shieldsbeing fixed to the shield plate.
 3. The system of claim 1 wherein thereare at least three targets arranged around the generally circular firstplate.
 4. The system of claim 1 wherein the cross-contamination shieldsterminate less than 10 mm from workpieces supported by the pallet. 5.The system of claim 1 wherein the cross-contamination shields terminateless than 3 mm from workpieces supported by the pallet.